Optical fiber and optical transmission path that uses the optical fiber

ABSTRACT

There is provided at low cost an optical fiber suitable for wavelength division multiplex transmissions that has strengthened the tolerance to bending loss at even smaller bending diameters. The present invention is an optical fiber whose base material is silica glass and that has a two layer structure formed by a core that has a substantially uniform refractive index and by a cladding that is located outside the core and that has a substantially uniform refractive index, wherein the optical fiber satisfies the following conditions (1) to (3): (1) an outer diameter of the core is in a range of 4 to 8 μm, and a relative refractive index difference of the core is in a range of 0.4 to 0.8%; (2) chromatic dispersion at a wavelength of 1550 nm is in a range of 2 to 15 ps/nm/km; and (3) effective area at a wavelength of 1550 nm is 40 μm 2  or more.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical fiber and to atransmission path that uses the optical fiber.

[0003] 2. Description of the Related Art

[0004] Corresponding to increases in the volume of data traffic,increases in the transmission capacity of networks are required.

[0005] For example, wavelength division multiplexing (WDM) system is atransmission system which is designed to meet this requirement and isalready available commercially.

[0006] Moreover, in recent years, investigations have been made intousing WDM transmissions not only in long haul systems, but also in metroand access systems.

[0007] WDM transmission systems, which are considered applicable to ametro and access system, can be broadly categorized into two types. Onetype is known as dense WDM (DWDM). This type is almost the same as longhaul systems.

[0008] The other type is known as coarse WDM (CWDM). This type is amethod that spreads each signal wavelength interval uses wide wavelengthband.

[0009] In WDM transmissions over a relatively short distance (distancesup to approximately 200 km may be included in the definition “relativelyshort distance”), such as in the case with a metro or access system, thesystem cost is extremely important factor to design the system. As aresult, it is necessary for the optical fiber used in such atransmission path to be low in price.

[0010] In the case of the installation of optical fiber to private homesand offices (Fiber to the Home—FTTH), in addition to low price, there isanother requirement which is required to the fiber for the abovedescribed WDM transmission path. Namely, when a fiber is laid in abuilding or private home, there is a possibility that the optical fiberwill be conditioned under extremely small bending of the order of 30 φmmor 20 φmm.

[0011] Furthermore, it is extremely important that there is no lossincrease even if the fiber is coiled in a small bending diameter inorder to store surplus fiber. Therefore, the fiber to the FTTH should betolerant to the bend.

[0012] In contrast, conventionally, several types of optical fiber havebeen proposed that are suitable for WDM. However, all of them have, forexample, three or more layers of complicated refractive index profileand are expensive because of such complicated structure.

[0013] Moreover, conventional 1.3 μm band single mode fibers ormultimode fibers are generally used in offices and homes,conventionally. However, these fibers generally only tolerate a bendingdiameter of approximately 60 mm. Consequently, when the fiber is beinglaid out careful attention is needed to ensure that there is noexcessive bending. Recently, fibers have become commercially availablethose are based on ITU-T G. 652, which is an international standard for1.3 μm band single mode fibers, and tolerate a bending diameter of 30 mmby reducing the size of the mode field diameter (MFD). However, theadditional development is awaited of an optical fiber that can cope withstill smaller bending diameters for wiring in buildings and privatehomes.

[0014] The present invention was conceived in view of the abovecircumstances and it is an object thereof to provide a low price opticalfiber that is suitable for WDM transmission.

[0015] A second object is to provide an optical fiber capable of beinglaid with a small bending diameter or of allowing surplus fiber thereofto be stored in small diameter.

SUMMARY OF THE INVENTION

[0016] In order to clear up the above problems the present inventorsdecided to investigate whether it was possible to obtain characteristicssuitable for WDM transmission using an optical fiber having a refractiveindex profile as simple as possible.

[0017] As a result, the present inventors discovered that it waspossible to obtain an optical fiber having optical characteristicssuitable for WDM transmission by using what is known as a step indexprofile with a two layer structure formed by a core and a cladding,which is considered the most simple structure, and has conventionallybeen considered unsuitable for WDM transmission.

[0018] Namely, the first aspect of the present invention is an opticalfiber whose base material is silica glass and that has a two layerstructure formed by a core that has a substantially uniform firstrefractive index and by a cladding that is located outside the core andthat has a substantially uniform second refractive index, wherein theoptical fiber satisfies the following conditions (1) to (3): (1) anouter diameter of the core is in a range of 4 to 8 μm, and a relativerefractive index difference between the first refractive index andsecond refractive index when second refractive index is taken as areference, is in a range of 0.4 to 0.8%; (2) chromatic dispersion at awavelength of 1550 nm is in a range of 2 to 15 ps/nm/km; and (3)effective area at a wavelength of 1550 nm is 40 μm² or more.

[0019] The second aspect of the present invention is the optical fiberaccording to the first aspect, wherein the outer diameter of the core isin a range of 4.5 to 5 μm, and the chromatic dispersion is in a range of2 to 6 ps/nm/km.

[0020] The third aspect of the present invention is the optical fiberaccording to the first aspect, wherein the outer diameter of the core isin a range of 5 to 8 μm, and the chromatic dispersion is in a range of 6to 15 ps/nm/km.

[0021] The fourth aspect of the present invention is the optical fiberaccording to the third aspect, wherein the outer diameter of the core isin a range of 5 to 6.5 μm, and the chromatic dispersion is 6 to 10ps/nm/km.

[0022] The fifth aspect of the present invention is the optical fiberaccording to any of the first through fourth aspects, wherein therelative refractive index difference is in a range of 0.4 to 0.6%, andthe effective area is 50 μm or more.

[0023] The sixth aspect of the present invention is the optical fiberaccording to the third or fourth aspects, wherein the relativerefractive index difference is in a range of 0.4 to 0.5%, and theeffective area is 60 μm² or more.

[0024] The seventh aspect of the present invention is the optical fiberaccording to any of the first through sixth aspects, whereintransmission loss is 0.35 dB/km or less over a wavelength range from1360 to 1400 nm.

[0025] The eighth aspect of the present invention is the optical fiberaccording to any of the first through seventh aspects, whereintransmission loss is 0.40 dB/km or less over a wavelength range from1260 to 1625 nm.

[0026] The present inventors also focused on a parameter known asV_(core) and discovered that, by limiting this V_(core), it was possibleto suppress chromatic dispersion at a wavelength of 1550 nm.

[0027] Namely, the ninth aspect of the present invention is the opticalfiber according to the fourth aspect, wherein V_(core) is 15% μm² orless, when V_(core) is obtained by multiplying π by a value obtained byintegrating the sum of r and Δn (r) with r within a range of 0 tor_(core), with Δn (r) being the relative refractive index difference,and with r_(core) being the outermost radius of the core at a radius rof the optical fiber.

[0028] The tenth aspect of the present invention is the optical fiberaccording to the ninth aspect, wherein the relative refractive indexdifference is in a range of 0.4 to 0.6%, and the effective area is 50μm² or more.

[0029] The eleventh aspect of the present invention is the optical fiberaccording to the third aspect, wherein the relative refractive indexdifference is in a range of 0.51 to 0.59%, and the core diameter is in arange of 5.5 to 7.0 μm.

[0030] The twelfth aspect of the present invention is the optical fiberaccording to the eleventh aspect, wherein when the relative refractiveindex difference is taken as Δ, V_(core) is greater than (−17.25·Δ+25.2)and less than 20% μm².

[0031] The thirteenth aspect of the present invention is the opticalfiber according to twelfth aspect, wherein the optical fiber has a modefield diameter (MFD) of 7.8 μm or greater at a wavelength of 1550 nm,and a bending loss of 0.3 dB/m or less at a bending diameter of 20 mm.

[0032] The fourteenth aspect of the present invention is the opticalfiber according to thirteenth aspect, wherein connection loss with anormal single mode optical fiber at ITU-T G.652 Recommendation is 0.35dB or less at a wavelength of 1550 nm.

[0033] The fifteenth aspect of the present invention is the opticalfiber according to the ninth aspects, wherein transmission loss is 0.35dB/km or less over a wavelength range from 1360 to 1400 nm.

[0034] The sixteenth aspect of the present invention is the opticalfiber according to the ninth aspects, wherein transmission loss is 0.40dB/km or less over a wavelength range from 1260 to 1625 nm.

[0035] The seventeenth aspect of the present invention is an opticaltransmission path comprising any one of the optical fibers as claimed inthe first to sixteenth aspects.

[0036] The eighteenth aspect of the present invention is the opticaltransmission path according to the seventeenth aspect further comprisinga dispersion compensator that is combined with the optical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1A is an explanatory diagram showing the refractive indexprofile of the optical fiber of the present invention, and shows a crosssection when a cut is made at a right angle relative to the centralaxial direction.

[0038]FIG. 1B is an explanatory diagram showing the refractive indexprofile of the optical fiber of the present invention, and shows arefractive index profile seen from the side.

[0039]FIG. 2 is an explanatory diagram of an example for showing a stateof the refractive index profile of an actually manufactured opticalfiber of the present invention.

[0040]FIG. 3A is an explanatory diagram of an example for showing statesof the refractive index profile of an actually manufactured opticalfiber of the present invention.

[0041]FIG. 3B is an explanatory diagram of an example for showing statesof the refractive index profile of an actually manufactured opticalfiber of the present invention.

[0042]FIG. 4 is a graph showing a relationship between optical fiberconnection loss and mode field diameter (MFD) at a wavelength of 1550nm.

[0043]FIG. 5 is a diagram showing a schematic construction of anembodiment of an optical transmission path.

[0044]FIG. 6 is a diagram showing a schematic construction of anembodiment of an optical transmission path with a dispersioncompensator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] The present invention will now be described in detail.

[0046] (The Optical Fiber of the First Aspect of the Present Invention)

[0047]FIGS. 1A and 1B show examples of refractive index profile of theoptical fiber of the present invention. FIG. 1A shows a cross sectionwhen a cut is made at a right angle relative to the central axialdirection. FIG. 1B shows a refractive index profile seen from the side.

[0048] The descriptor 1 in the drawings is a core. Cladding 2 isprovided in a concentrically circular form on the outside of the core 1,and an optical fiber is formed from the two layers of the core 1 and thecladding 2.

[0049] The core 1 and the cladding 2 both have substantially uniformrefractive indexes, however, the refractive index of the core 1 ishigher than that of the cladding 2.

[0050] The base material of this optical fiber is silica glass and thematerials of the core 1 and cladding 2 are decided based on desirabletheir refractive indexes. For example, if the core 1 is formed fromgermanium doped silica glass or the like, the cladding 2 is formed frompure silica glass or from fluorine doped silica glass or the like. Ifthe core 1 is formed from pure silica glass or the like, the cladding 2is formed from fluorine doped silica glass or the like.

[0051] In an optical fiber that has silica glass as the base material,Rayleigh scattering loss is reduced if there is only a small amount ofdopant such as germanium and fluorine added thereto. Therefore, it ispreferable that only one of the core 1 and the cladding 2 is formed froma material that has dopant added thereto and the other is formed fromsubstantially pure silica glass. Accordingly, it is preferable that, forexample, the core 1 is formed from germanium doped silica glass and thecladding 2 is formed from substantially pure silica glass, or that thecore 1 is formed from substantially pure silica glass and the cladding 2is formed from fluorine doped silica glass.

[0052] In the meanwhile, important properties those are required to thefibers for WDM systems, and particularly for DWDM systems, are chromaticdispersion, effective area and so on.

[0053] At this time, the fibers should be satisfied the characteristics,which are required foran conventional SM fiber, such as the polarizationmode dispersion (PMD), cutoff wavelength, and bending loss. Although notparticularly restricted, it is preferable that the PMD is 0.1 ps/{squareroot}{square root over ( )}km or less, that the cutoff wavelength is avalue that allows single mode transmission in the wavelengths used fortransmission, and that at least the cutoff wavelength measured at thelength which would be in actual use is shorter than the minimumwavelength used for transmission.

[0054] The wavelengths used for transmission are users definable, forexample, from what is known as the S band, to the C band, and to the Lband (1460 nm to 1625 nm). In the optical fiber of the presentinvention, a value of 1.4 μm or less is obtained for the cable cutoffwavelength. Cable cutoff wavelength is described in ITU-T RecommendationG. 650. Moreover, a value of 10 dB/m or less is obtained for the bendingloss in measurement conditions of a bending diameter of 20 mm at awavelength of 1550 nm, for example.

[0055] The smaller the absolute value of the chromatic dispersion, thehigher the four wave mixing (FWM) efficiency. FWM is a cause of signaldistortion in WDM transmission, and particularly, in DWDM. Therefore,the efficiency is preferable to be suppressed.

[0056] Because of this, it is desirable that an optical fiber used forWDM has a chromatic dispersion of at least 1 ps/nm/km or more in thewavelength range used for transmission, and preferably a wavelengthdispersion of 2 ps/nm/km or more.

[0057] On the other hand, if the chromatic dispersion is too large,signal distortion caused by self phase modulation (SPM) and by thedispersion becomes problematic. For example, dispersion compensationbecomes necessary which leads to increased costs, or, the transmissiondistance is limited by the distortion. Therefore, it is desirable thatthe chromatic dispersion of the optical fiber used for WDM is small.

[0058] Because of these two antipodal requirements, in WDM transmission,an optical fiber having an intermediate chromatic dispersion that is nottoo large and not too small is desirable.

[0059] In order to satisfy the above requirements over a wider range, asmall dispersion slope over the wavelength range where is used fortransmission is desired.

[0060] A large effective area (Aeff) is preferred so that non-lineareffects such as SPM, FWM, and the like can be suppressed.

[0061] The optical fiber of the first aspect of the present inventionsatisfies the conditions (1) to (3) above, and can satisfy theconditions of chromatic dispersion and effective area required to anoptical fiber used in relevant WDM transmissions and, preferably, inDWDM transmissions.

[0062] Concerning (1) above, the outer diameter of the core is 4 to 8μm. If the outer diameter of the core is less than 4 μm the chromaticdispersion is too small, as a result the fiber is insufficient for FWMsuppression. If the outer diameter of the core exceeds 8 μm thechromatic dispersion is too big and the transmission characteristics aredeteriorated.

[0063] The relative refractive index difference of the core 1 is 0.4 to0.8%. The relative refractive index difference of the core 1 is definedby (n1²−n2²)/2n1² where the refractive index of the core 1 is n1 and therefractive index of the cladding 2 is n2.

[0064] If the relative refractive index difference of the core 1 is lessthan 0.4%, then the bending loss becomes too high when an optical fiberis designed to have the aforementioned intermediate chromatic dispersionand the optical fiber cannot put to actual use. If the relativerefractive index difference of the core 1 exceeds 0.8% then theeffective area becomes so small as to cause the problematic level ofnon-linear effect. Note that the relative refractive index differencecan be adjusted, for example, by changing the amount of dopant that isdoped during manufacturing.

[0065] The optical fiber of the present invention can be obtained byselecting a combination of the relative refractive index difference andthe core diameter from the range of 4 to 8 μm in outer diameter of core1 and the range of 0.4 to 0.8% in relative refractive index differenceof core 1 which are the conditions of (1) above, that satisfies theabove mentioned optical characteristics of (2) and (3) throughsimulation. The simulation method may be a technique commonly used bypeople skilled in the art.

[0066] Here, in so-called step index profile, the dispersion slope isknown to be substantially constant between approximately 0.05 to 0.06ps/nm²/km, and this range is favorable because the above describedintermediate chromatic dispersion value can be kept over a widewavelength range. Therefore, the fiber can use in WDM transmissions and,particularly, DWDM transmissions over a wide wavelength range.

[0067] Concerning (2) above, if the chromatic dispersion at a wavelengthof 1550 nm is 2 ps/nm/km or more then FWM can be suppressed even, forexample, within what is known as the C band (1530 to 1565 nm). If thewavelength dispersion at a wavelength of 1550 nm is 15 ps/nm/km or less,then even at an upper limit wavelength (1625 nm) of the L band (1565 to1625 nm), the chromatic dispersion value is approximately 18 ps/nm/km orless. Therefore the usable wavelength band can be extended to the upperlimit value of the L band.

[0068] Because the chromatic dispersion at a wavelength of 1550 nm of aconventional single mode optical fiber for 1.3 μm use is approximately18 ps/nm/km, from the standpoint of chromatic dispersion, if the opticalfiber of the present invention is used, then even if the wavelengthrange is extended to 1625 nm, transmission characteristics that aresubstantially the same as for a wavelength of 1550 nm can be obtained,which is preferable from the standpoints of enlarging the applicablewavelength range for wavelength multiplexing.

[0069] If the effective area satisfies the range of (3) above (40 μm² ormore and, essentially, from the standpoint of the limits of othercharacteristics such as bending loss, as well as in viewpoint ofmanufacturability, 70 μm² or less) then non-linear optical effect can besuppressed and transmission characteristics improved.

[0070] Here, usually, the outer diameter of the cladding 2 is setapproximately 125 μm.

[0071] One type of an optical fiber provided with a two layer of a coreand a cladding (what is known as a step index profile), isconventionally known as, for example, a 1.3 μm single mode opticalfiber. However, heretofore, an optical fiber provided with a step indexprofile was considered to not be suitable for WDM transmissions and hasnot attracted attention as an optical fiber for WDM transmissions.

[0072] In a conventional optical fiber provided with a step indexprofile (a 1.3 μm single mode optical fiber), the outer diameter of thecore of the fiber and the relative refractive index difference of it donot satisfy the conditions of (1) above, and it can also not providesuch characteristics as shown by (2) and (3). Accordingly, if this typeof conventional standard single mode optical fiber for 1.3 μm is usedfor WDM transmissions and, in particular, for DWDM transmissions,problems arise such as the transmission distance being limited or thecost of the overall system being raised due to an increased value ofchromatic dispersion that needs to be compensated by a dispersioncompensator. Consequently, the structure and effects of a conventionalstandard single mode optical fiber for 1.3 μm and the optical fiber ofthe present invention are completely different.

[0073] (The Optical Fiber of the Second Aspect of the Present Invention)

[0074] The optical fiber of the second aspect of the present inventionis the optical fiber of the first aspect of the present invention inwhich the outer diameter of the core 1 is set to within a range of 4.5to 5 μm, thereby enabling chromatic dispersion at a wavelength of 1550nm to be set in a range of 2 to 6 ps/nm/km. This allows transmissionloss to be reduced, and enables transmission characteristics to beimproved.

[0075] Moreover, because the wavelength dispersion value in the L bandscan be suppressed this gives the advantage of being good transmissioncharacteristics in the L band.

[0076] If chromatic dispersion at a wavelength of 1550 nm is 6 ps/nm/km,for example, in the C band 10G transmission is possible over a distanceof 100 km or more without dispersion compensation.

[0077] Accordingly, it is preferable that the optical fiber of thesecond aspect of the present invention is used in a wavelength bandselected from the C to L bands.

[0078] (The Optical Fiber of the Third Aspect of the Present Invention)

[0079] The optical fiber of the third aspect of the present invention isthe optical fiber of the first aspect of the present invention in whichthe outer diameter of the core 1 is set to within a range of 5 to 8 μm,thereby enabling chromatic dispersion at a wavelength of 1550 nm to beset in a range of 6 to 15 ps/nm/km.

[0080] Because of such characteristics, it is possible to suppress FWMin a shorter wavelength region, namely, in what is known as the S band(1460 to 1530 nm). If the diameter of the core 1 is less than 5 μm,chromatic dispersion in the S band is decreased and it is not possibleto suppress FWM.

[0081] If chromatic dispersion at a wavelength of 1550 nm is 6 ps/nm/km,a high chromatic dispersion (preferably approximately 1 ps/nm/km ormore) that makes it possible to suppress FWM is assured even, forexample, at the lower limit wavelength (1460 nm) of the S band.

[0082] Accordingly, it is preferable that the optical fiber of the thirdaspect of the present invention is used in a wavelength band selectedfrom the S to C bands.

[0083] (The Optical Fiber of the Fourth Aspect of the Present Invention)

[0084] The optical fiber of the fourth aspect of the present inventionis the optical fiber of the third aspect of the present invention inwhich the outer diameter of the core 1 is further set to within a rangeof 5 to 6.5 μm, thereby enabling chromatic dispersion at a wavelength of1550 m to be set in a range of 6 to 10 ps/nm/km.

[0085] The optical fiber of the fourth aspect of the present inventionis suitable for use in the S band and further reduces the upper limitvalue of the chromatic dispersion while simultaneously improvingtransmission characteristics in the L band.

[0086] Accordingly, it is preferable that the optical fiber of thefourth aspect of the present invention is used in a wavelength bandselected from the S to C to L bands.

[0087] (The Optical Fiber of the Fifth Aspect of the Present Invention)

[0088] The optical fiber of the fifth aspect of the present invention isthe optical fiber of any of the first through fourth aspects in whichthe relative refractive index difference of the core 1 is set to withina range of 0.4 to 0.6%, thereby enabling the effective area at awavelength of 1550 nm to be set to 50 μm² or more (essentially, from thestandpoint of the limits of other characteristics such as bending loss,as well as in view of manufacturability, 70 μm² or less). As a result,the effect of non-linear optical effect is further suppressed.

[0089] (The Optical Fiber of the Sixth Aspect of the Present Invention)

[0090] The optical fiber of the sixth aspect of the present invention isthe optical fiber of the fifth aspect in which the relative refractiveindex difference of the core 1 is set to 0.4 to 0.5%, thereby enablingthe effective area to be set to 60 μm² or more (essentially, from thestandpoint of the limits of other characteristics such as bending loss,as well as in view of manufacturability, 70 μm² or less). As a result,the effect of non-linear optical effect is further suppressed.

[0091] At this time, it is preferable adjusting to be made such that thechromatic dispersion is in the range of 6 to 15 ps/nm/km. If theconditions are satisfied with the chromatic dispersion in a range of 2ps/nm/km or more and less than 6 ps/nm/km, at a wavelength of 1550 nm,bending loss at a bending diameter of 20 mm tends to increase, forexample, to 40 dB/m or more, therefore, the fiber can not put to actualuse.

[0092] (The Optical Fiber of the Seventh Aspect of the PresentInvention)

[0093] One of required characteristics for the fiber for CWDMtransmission is low transmission loss over a wide wavelength regionbecause of the fact that an optical amplifier does not use in CWDMtransmission.

[0094] The optical fiber of the seventh aspect of the present inventionis the optical fiber of any of the first through sixth aspects in whichtransmission loss over the wavelength range from 1360 to 1400 nm is 0.35dB or less and, preferably, 0.32 dB or less. Accordingly, this opticalfiber can be used in any WDM transmission (including CWDM and DWDMtransmissions).

[0095] Because an absorption peak caused by Si—OH bonds formed withinthe silica glass (in the vicinity of 1380 nm) is present, it is knownthat transmission loss becomes worse within this wavelength range. Si—OHbonds are formed by OH group contamination during the optical fibermanufacturing process. They can be removed to a certain extent, forexample, by using a dehydrating agent. Because, however, it is notpossible to remove them completely, it is desirable that OH groupcontamination be held to as low a level as possible.

[0096] If a conventional optical fiber for WDM having a complicatedrefractive index profile is used, as the manufacturing process increasesto make such complicated profile, there is increased opportunity for OHgroup contamination to be pitched during the optical fiber manufacturingprocess, as a result, it is difficult to sufficiently reduce absorptioncaused by Si—OH bonds.

[0097] However, because the optical fiber of the present invention has asimple refractive index profile, it is relatively easy to prevent Si—OHbonds being formed in the optical fiber during the manufacturingprocess. As a result, low transmission loss is obtained across the 1360to 1400 nm wavelength range as is described above.

[0098] (The Optical Fiber of the Eighth Aspect of the Present Invention)

[0099] The core 1 of the optical fiber of the present invention has alow relative refractive index difference compared with conventionaloptical fibers for WDM transmission that have a complicated refractiveindex profile. Therefore, in the case of the optical fiber of thepresent invention, the amounts of dopants such as germanium and fluorinethat are doped can be reduced. This enables Rayleigh scattering loss tobe reduced.

[0100] The amount of dopant and the like is appropriately adjusted inaccordance with the design, manufacturing conditions and the like. Forexample, if the core 1 is formed from germanium doped silica glass andthe cladding 2 is formed from pure silica glass,, the amount of dopedgermanium may be, for example, 4.0 to 7.5 mol % in germanium oxideequivalent weight (equivalent to a relative refractive index differenceof 0.4 to 0.8%).

[0101] Moreover, in addition to the effect of being able to reduceoptical absorption caused by the aforementioned Si—OH bonds, an opticalfiber having a transmission loss of 0.40 dB/km or less and preferably0.36 dB/km or less over the 1260 to 1625 nm wavelength range can beobtained.

[0102] If an optical fiber is actually manufactured based on an idealrefractive index profile design such as shown in FIGS. 1A and 1B using aconventionally known method such as the MCVD, OVD, or VAD methods, forexample, “sagging” or “horns” in the refractive index may be generatedin the core 1, as is shown in the example in FIG. 2, which is caused byfluctuations and the like during manufacturing in some cases.

[0103] However, essentially, these do not particularly pose problems ifthe value of (1) above is satisfied in an equivalent manner. Forexample, in an actually manufactured optical fiber it is sufficient if avalue obtained by averaging values of refractive index and the like ofthe core satisfies the refractive index profile conditions of the idealrefractive index profile.

[0104] As described above, an example of the term “provided that,essentially, the design condition for (1) above is satisfied in anequivalent manner” is given in the following case.

[0105] Firstly, possible examples for the relative refractive index ofthe core are given below. (a) A case when, in an actually manufacturedoptical fiber, the average value of the refractive index of the corethereof satisfies refractive index profile conditions of an idealrefractive index profile (a step refractive index profile) described in(1) above, and also the optical characteristics of the optical fibersatisfy optical characteristics such as those described above. (b) It issufficient if, essentially, the refractive index profile is unimodal.This includes all cases other than those in which the refractive indexprofile is intentionally formed as not unimodal.

[0106] Examples of this include: as is shown in FIG. 3A, cases in whichthe average value of the core refractive index satisfies the refractiveindex profile conditions described in (1) above, and fluctuations in thecore refractive index are within ±25% of the average value of the corerefractive index; and, as is shown in FIG. 3B, cases in which theaverage value of the core refractive index satisfies the refractiveindex profile conditions described in (1) above, and there is an abruptfluctuation in the refractive index within a diametrical range of 2 μmfrom the core center (a core refractive index having an unintentionalfluctuation that appears when a collapsing is being performed in theMCVD and PCVD methods).

[0107] For the outer diameter of the core, an average value iscalculated for the core refractive index and, in an ideal refractiveindex profile (a step refractive index profile) for this average value,the outer diameter is acceptable if it is within the numerical range forthe core outer diameter described in (1) above, and also if the opticalcharacteristics are satisfied. Essentially, for example, in the actualrefractive index profile, the outer diameter is acceptable if a rangehaving a refractive index of 20 to 80% of the average value of the corerefractive index is within the range of the core outer diameterdescribed in (1) above.

[0108] Here, the optical fiber of the present invention can bemanufactured using a conventional known method.

[0109] (The Optical Fiber of the Ninth Aspect of the Present Invention)

[0110] In the optical fiber of the above described first to eighthaspects, a profile of an optical fiber that provides opticalcharacteristics appropriate for WDM transmission (an Aeff ofapproximately 50 μm² at intermediate dispersion) while having a simplestructure (a step profile) is mentioned.

[0111] In the optical fiber of the above described first to eighthaspects of the present invention the refractive index is stipulatedusing the parameters of the core diameter and the core relativerefractive index difference. In contrast, the optical fiber of the ninthaspect of the present invention a parameter called a V_(core) isintroduced to the optical fiber of the fourth aspect, and it is madeclear that an optical fiber having a chromatic dispersion of 10 ps/nm/kmcan be obtained by defining the relationship between the core diameterand the relative refractive index difference and by setting V_(core) to15% μm² or less.

[0112] The parameter known as V_(core) is defined by Formula (1). Theunit thereof is % μm². Here, Δn (r) is the relative refractive indexdifference at the radius r when the refractive index of the cladding ofthe optical fiber is taken as a reference, and r_(core) is the outermostradius of the core. $\begin{matrix}{V_{core} = {\pi {\int_{0}^{r_{core}}{{{\Delta_{n}(r)} \cdot r}\quad {r}}}}} & (1)\end{matrix}$

[0113] Namely, V_(core) can be obtained by multiplying π by a valueobtained by integrating the sum of r and Δn (r) with r in a range of 0to r_(core).

[0114] By limiting V_(core) represented by Formula (1) to 15% μm² orless, it is possible to suppress the chromatic dispersion at awavelength of 1550 nm to +10 ps/nm/km or less. This relationship is alsoparticularly effective in the case of refractive index profiles whichhave sagging or horns in the refractive index profile, as is shown inFIGS. 2 and 3.

[0115] (The Optical Fiber of the Tenth Aspect of the Present Invention)

[0116] The optical fiber of the tenth aspect of the present invention isthe optical fiber of ninth aspect in which the relative refractive indexdifference of the core 1 is set to within a range of 0.4 to 0.6%,thereby enabling the effective area at a wavelength of 1550 nm to be setto 50 μm² or more (essentially, from the standpoint of the limits ofother characteristics such as bending loss, as well as in view ofmanufacturability, 70 μm² or less). As a result, the effect ofnon-linear optical effect is further suppressed.

[0117] (The Optical Fiber of the Eleventh Aspect of the PresentInvention)

[0118] The optical fiber of the eleventh aspect of the present inventionis furnished with a mode field diameter of 7.8 μm or more, a cutoffwavelength of 1.26 μm or less, and also a bending loss of 0.3 dB/m orless at 20 φmm by setting the relative refractive index difference ofthe core 1 of the optical fiber of the third aspect of the presentinvention in a range between 0.51 and 0.59% and by also setting the corediameter thereof in a range between 5.5 and 7.0 μm. As a result, it ispossible to strengthen the tolerance to bending loss in a small bendingdiameter in comparison with a conventional single mode fiber for the 1.3μm band. Consequently, an optical fiber is obtained that can be laidwith a small bend diameter or whose surplus fiber can be stored in asmall diameter, which are features sought in an office or in the home.

[0119] If the relative refractive difference Δ is 0.51% or less it isnot possible to achieve both an λc of 1.26 μm or less and a bending lossof 0.3 dB/m or less. In optical transmission, the wavelength bandbetween 1.26 and 1.625 μm is widely used as a transmission band, and acutoff wavelength of 1.26 μm or less is necessary from the viewpoint ofapplicability to CWDM transmission systems.

[0120] In addition, if the relative refractive difference Δ is 0.59% ormore, the MFD at a wavelength of 1550 nm goes below 7.8 μm. If the MFDgoes below 7.8 μm then problems arise regarding the connectivity withconventional single mode optical fibers recommended in ITU-T G. 652.Conventional single mode optical fibers are widely used fortransmission, and the nominal value of the MFD in ITU-T G.652 isrestricted to 8.6 to 9.5 μm at 1310 nm. A conventional single modeoptical fiber having an MFD of approximately 9.2 μm at 1310 nm has anMFD of approximately 10.4 μm at 1550 nm. Therefore, as described above,if the MFD goes below 7.8 μm there is a concern that connection losswith a conventional single mode optical fiber will exceed 0.35 dB.

[0121] Such increase in connection loss is undesirable as it causes areduction in the loss margin of a designed system. For example, in theGeneral Conference of the Institute of Electronic, Information andCommunication Engineers B-10-29 for the year 2000, investigations weremade into an optical fiber for laying indoors that was resistant tobending. According to these investigations, a dispersion shift opticalfiber (referred to below as Recommendation ITU-T G.653) is excellentwith regard to bend characteristics, however, connection loss with aconventional single mode optical fiber is approximately 0.55 dB at awavelength of 1550 nm, and approximately 0.87 dB at a wavelength of 1310nm, which, it was indicated, is problematic.

[0122] In contrast to this, the optical fiber of the present inventionis desirable because a connection loss of 0.35 dB or less could beobtained in approximately half the results of the above investigation,which solves the above described practical problems.

[0123] (The Optical Fiber of the Twelfth Aspect of the PresentInvention).

[0124] The optical fiber of the twelfth aspect of the present inventionis the optical fiber of the eleventh aspect in which V_(core) is setgreater than (−17.25·Δ+25.2) and less than 20% μg m² when the relativerefractive index difference is Δ. Namely, this relationship is shown byFormula (2).

−17.25×Δ+25.2<V _(core) <20  (2)

[0125] The relationship shown in Formula (2) is a relational expressiondetermined from results of the Examples described below. V_(core) and Δboth satisfy the above Formula (2) in the above described optical fiberof the eleventh aspect. This fact is also necessary to obtain a bendingloss of 0.3 dB/m or less. If the V_(core) exceeds 20% μm² then itbecomes difficult to achieve λc of 1.26 μm or less.

[0126] (The Optical Fiber of the Thirteenth Aspect of the PresentInvention).

[0127] The optical fiber of the thirteenth aspect of the presentinvention is the optical fiber of the twelfth aspect in which the modefield diameter (MFD) at a wavelength of 1550 nm is set to 7.8 μm ormore, and the bending loss at a bending diameter of 20 mm is set to 0.3dB/m or less.

[0128] According to this aspect, as described above, the optical fiberof the thirteenth aspect of the present invention has a mode fielddiameter MFD at a wavelength of 1550 nm of 7.8 μm or more. Therefore,when the optical fiber of the thirteenth aspect is connected to aconventional single mode optical fiber, it is possible to keepconnectability at the excellent level of 0.35 dB or less. Moreover, bysetting the bending loss at a bending diameter of 20 mm to 0.3 dB/m orless it is possible to keep any increase in loss to an extremely smalllevel even if the fiber is bent at 20 φmm. As a result,, the opticalfiber of the thirteenth aspect has excellent characteristics forapplications such as indoor wiring, in whiche a small tolerable bendingdiameter is sought after.

[0129] (The Optical Fiber of the Fourteenth Aspect of the PresentInvention).

[0130] The optical fiber of the fourteenth aspect of the presentinvention is the optical fiber according to the thirteenth aspect inwhich connection loss at a wavelength of 1550 nm is 0.3 dB or less.

[0131] Typical examples of connection loss in optical fibers can befound in the paper “Loss Analysis of Single Mode Fiber Splices”, BellSyst. Tech. J., Vol. 56, No. 5, P. 703, May 1997 by D. Marcuse. In thispublication connection loss in optical fibers having different modefield diameters (MFD) is defined using Formula (3) below. Here, Tg isconnection loss, 2w₁ and 2w₂ are the mode field diameters of eachoptical fiber being connected, and d is the amount of axialmisalignment. $\begin{matrix}{T_{g} = {\left( \frac{2w_{1}w_{2}}{w_{1}^{2} + w_{2}^{2}} \right)^{2}{\exp \left( {- \frac{2d^{2}}{w_{1}^{2} + w_{2}^{2}}} \right)}}} & (3)\end{matrix}$

[0132] If an ideal connection with no axial misalignment is supposed,Formula (3) above can be abbreviated to Formula (4) below.$\begin{matrix}{T_{g} = \left( \frac{2w_{1}w_{2}}{w_{1}^{2} + w_{2}^{2}} \right)^{2}} & (4)\end{matrix}$

[0133] The MFD at 1550 nm of a conventional single mode optical fiberrecommended in ITU-T G. 652 recommendation is approximately 10.4 μm.FIG. 4 shows the results when the MFD (=2w₁) dependency of theconnection loss for a conventional single mode optical fiber (2w₂=10.4μm) is calculated using Formula (4). From FIG. 4 it can be seen that theconnection loss increases as the MFD) difference relative to aconventional single mode optical fiber increases.

[0134] In contrast to this, as is shown by the thirteenth aspect, theoptical fiber of the present invention has an MFD of 7.8 μm or more at awavelength of 1550 nm. Consequently, the connection loss with aconventional single mode optical fiber at ITU-T G. 652 recommendation is0.35 dB or less at a wavelength of 1550 nm, which is extremely desirablefor practical applications.

[0135] (The Optical Fiber of the Fifteenth Aspect of the PresentInvention).

[0136] One of required characteristics for the fiber for CWDMtransmission is low transmission loss over a wide wavelength regionbecause of the fact that an optical amplifier does not use in CWDMtransmission.

[0137] The optical fiber of the fifteenth aspect of the presentinvention is the optical fiber of ninth aspect in which transmissionloss over the wavelength range from 1360 to 1400 nm is 0.35 dB or lessand, preferably, 0.32 dB or less. Accordingly, this optical fiber can beused in any WDM transmission (including CWDM and DWDM transmissions).

[0138] (The Optical Fiber of the Sixteenth Aspect of the PresentInvention).

[0139] The optical fiber of the fifteenth aspect of the presentinvention is the optical fiber of ninth aspect having a transmissionloss of 0.40 dB/km or less and preferably 0.36 dB/km or less over the1260 to 1625 nm wavelength range.

[0140] (The Optical Transmission Path (the Seventeenth and EighteenthAspects of the Present Invention))

[0141] The seventeenth aspect of the present invention is an opticaltransmission path that comprises the optical fiber of any of the firstthrough sixteenth aspects.

[0142]FIG. 5 is a diagram showing a schematic construction of anembodiment of the optical transmission path.

[0143] If the optical fiber of the present invention is used an opticaltransmission path for WDM (including CWDM and DWDM) transmissions can beprovided at low cost.

[0144] In this optical transmission path, it is possible to use adispersion compensator that compensates chromatic dispersion of theoptical fiber of the present invention. Employing this aspect ispreferable as it enables transmission loss to be reduced even further.

[0145]FIG. 6 is a diagram showing a schematic construction of anembodiment of the optical transmission path with a dispersioncompensator.

[0146] The construction of the dispersion compensator is notparticularly restricted. For example, it is sufficient if a dispersioncompensating fiber is inserted into a location on the transmission path.It is also possible to employ a module type of dispersion compensator orthe like that uses fiber grating or a dispersion compensating opticalfiber or the like. Moreover, especially, a module type of dispersioncompensator that can also compensate a dispersion slope (known as adispersion slope compensating type) is preferable. It is also possible,for example, to install dispersion compensators suitable for each of theS, C, and L bands.

[0147] Moreover, it is also possible, for example, to install opticalamplifier(s) before or (and) after the dispersion compensator.

[0148] The value of the chromatic dispersion after it has beencompensated by a dispersion compensator can be adjusted, for example, byaltering the length of the dispersion compensation optical fiber or byaltering the value of the chromatic dispersion for each unit when adispersion compensation optical fiber is used, and it is preferable thata dispersion compensator is used that compensates the chromaticdispersion within the wavelength region that is used to in a range of −1to 1 ps/nm/km, and more preferably to in a range of −0.2 to 0.2ps/nm/km.

[0149] In the present invention, because it is possible in this mannerto use a simple refractive index profile, control of the structuralparameters and the like during the manufacturing process is simplified,therefore, the production yield can be improved. It is also possible toperform use a comparatively simple manufacturing apparatus because ofits simple refractive index profile.

EXAMPLES

[0150] The present invention will now be described in detail withexamples given.

[0151] Optical fibers having a two layer structure formed by a core anda cladding in which the outer diameter of the core (denoted in thetables as “Core Diameter”) and the relative refractive index differenceof the core (denoted in the tables as “Relative Refractive IndexDifference”) are shown in TABLES 1 to 3 were manufactured using a VADmethod. The cores were formed from germanium doped silica glass whilethe claddings were formed from substantively pure silica glass.

[0152] The optical characteristics of the manufactured optical fibershave been shown in TABLES 1 to 3. Here, unless it is noted otherwise,the measured values were measured at a wavelength of 1550 nm. TheV_(core) shown in TABLES 1 to 3 is the V_(core) defined by Formula (1)above. TABLE 1 Example Property (Unit) 1 2 3 4 5 6 7 Core Diameter (μm)5.4 4.9 5.5 6.4 5.1 5.9 7.0 Relative Refractive 0.62 0.66 0.52 0.50 0.550.44 0.44 Index Difference Δ (%) V_(core) 14.20 12.45 12.35 16.08 11.2412.03 16.93 (% μm²) Chromatic Dispersion 8.1 4.0 7.3 12.8 3.9 9.5 13.9(ps/nm/km) Chromatic Dispersion 0.054 0.054 0.056 0.056 0.057 0.0570.057 Slope (ps/nm²/km) Effective Area Aeff 42.8 40.4 51.4 53.6 50.361.4 61.5 (μm²) Bending Loss 0.5 0.8 3.1 0.7 4.2 6.4 0.8 (dB/m)@ 20 φmmCutoff Wavelength 1.07 1.04 1.05 1.11 1.25 1.01 1.15 (μm) @ ITU-T G. 650PMD (ps/{square root}km) 0.03 0.02 0.01 0.04 0.04 0.01 0.02 TransmissionLoss 0.192 0.197 0.191 0.189 0.193 0.186 0.188 (dB/km) @ 1550 nmTransmission Loss 0.32 0.33 0.30 0.34 0.35 0.30 0.29 (dB/km) @ max 1360nm to 1550 nm Transmission Loss 0.39 0.38 0.36 0.37 0.39 0.37 0.36(dB/km) @ max 1260 nm to 1625 nm

[0153] TABLE 2 Example Property (Unit) 8 9 10 11 12 13 14 Core Diameter(μm) 6.9 6.9 6.6 6.8 6.5 6.4 6.6 Relative Refractive 0.51 0.52 0.52 0.540.54 0.54 0.56 Index Difference Δ (%) V_(core) (% μm²) 19.1 19.3 18.019.4 18.0 17.1 19.0 Chromatic Dispersion 14.1 14.1 13.3 13.9 12.9 12.313.3 (ps/nm/km) Chromatic Dispersion 0.057 0.057 0.056 0.056 0.056 0.0560.056 Slope (ps/nm²/km) Effective Area Aeff 53.31 53.39 52.54 51.5350.57 50.09 49.35 (μm²) Mode Field Diameter 8.42 8.34 8.30 8.19 8.148.12 8.03 MFD (μm) Bending Loss 0.20 0.12 0.26 0.05 0.16 0.24 0.04(dB/m)@ 20 φmm Cutoff Wavelength 1.24 1.25 1.21 1.26 1.21 1.18 1.25 (μm)@ ITU-T G. 650 PMD (ps/{square root}km) 0.03 0.04 0.03 0.05 0.01 0.030.03 Transmission Loss 0.189 0.192 0.191 0.191 0.189 0.191 0.190 (dB/km)@ 1550 nm Transmission Loss 0.32 0.33 0.33 0.32 0.33 0.32 0.33 (dB/km) @max 1360 nm to 1550 nm Transmission Loss 0.38 0.39 0.38 0.37 0.37 0.380.36 (dB/km) @ max 1260 nm to 1625 nm

[0154] TABLE 3 Example Property (Unit) 15 16 17 18 19 20 Core Diameter6.4 6.1 6.5 6.3 5.9 6.3 (μm) Relative 0.56 0.56 0.58 0.58 0.58 0.59Refractive Index Difference Δ (%) V_(core) (% μm²) 18.0 16.2 19.3 18.015.6 18.4 Chromatic 12.6 11.1 13.2 12.3 10.2 12.5 Dispersion (ps/nm/km)Chromatic 0.055 0.055 0.056 0.055 0.055 0.051 Dispersion Slope(ps/nm²/km) Effective 48.75 47.86 47.83 47.05 46.00 46.49 Area Aeff(μm²) Mode Field 7.99 7.95 7.90 7.85 7.81 7.80 Diameter MFD (μm) BendingLoss 0.08 0.28 0.01 0.04 0.27 0.02 (dB/m)@ 20 φmm Cutoff 1.21 1.15 1.261.21 1.13 1.23 Wavelength (μm) @ ITU-T G. 650 PMD 0.04 0.05 0.03 0.020.02 0.03 (psk/{square root}km) Transmission 0.193 0.192 0.190 0.1920.193 0.192 Loss (dB/km) @ 1550 nm Transmission 0.34 0.33 0.31 0.32 0.320.34 Loss (dB/km)@ max 1360 nm to 1550 nm Transmission 0.37 0.37 0.390.38 0.38 0.37 Loss (dB/km)@ max 1260 nm to 1625 nm

[0155] TABLE 4 Example Property (Unit) 21 22 23 Core Diameter (μm) 6.57.5 7.5 Relative Refractive 0.50 0.50 0.46 Index Difference Δ (%)V_(core) (% m²) 16.6 22.1 20.3 Chromatic Dispersion 12.7 15.7 15.6(ps/nm/km) Chromatic Dispersion 0.056 0.057 0.058 Slope (ps/nm²/km)Effective Area Aeff 53.93 57.66 61.56 (μm²) Mode Field Diameter 8.438.62 8.93 MFD (μm) Bending Loss 1.06 0.06 0.46 (dB/m) @ 20 φmm CutoffWavelength 1.17 1.33 1.28 (μm) @ ITU-T G. 650 PMD (ps/{square root}km)0.02 0.03 0.03 Transmission Loss 0.189 0.191 0.190 (dB/km) @ 1550 nmTransmission Loss 0.33 0.35 0.34 (dB/km) @ max 1360 nm to 1550 nmTransmission Loss 0.38 0.39 0.39 (dB/km) @ max 1260 nm to 1625 nm

[0156] Each of the optical fibers shown in TABLES 1 to 3 has a chromaticdispersion at a wavelength of 1550 nm in a range of 2 to 15 ps/nm/km andan effective area at a wavelength of 1550 nm of 40 μm² or more, and isthereby suitable for use in WDM transmissions.

[0157] Furthermore, by comparing Examples 1, 2, 3, 5, and 6 in TABLE 1with the other examples it can be seen that chromatic dispersion at awavelength of 1550 nm can be restricted to +10 ps/nm/km or less bylimiting the V_(core) to 15% μm² or less.

[0158] Moreover, as is clear from the other examples (excepting Examples21 to 23), by setting the relative refractive index difference of thecore 1 to between 0.51 to 0.59%, and by also setting the core diameterto between 5.5 to 7.0 μm, it is possible to obtain a fiber having a modefield diameter of 7.8 μm or more and a cutoff wavelength of 1.26 μm orless as well as having a bending loss at 20 φmm of 0.3 dB/m or less.

[0159] In contrast, Examples 21 to 23 in TABLE 4 show cases in whichboth the relative refractive index difference and the core diameter areoutside the above described ranges. The example show that it is notpossible to obtain a fiber that simultaneously has a cutoff wavelengthof 1.26 μm or less as well as a bending loss at 20 μmm of 0.3 dB/m orless.

[0160] As has been described above, in the present invention, because itis possible to use a simple refractive index profile, it is possible toprovide at low cost an optical fiber suitable for WDM transmissions anda transmission path that uses this optical fiber.

[0161] Moreover, by limiting the V_(core) of the present invention to15% μm² or fewer, not only can the above described cost reduction beachieved, but an optical fiber can be provided with strengthened thetolerance to bending loss even in a bending diameter in comparison witha conventional single mode fiber for the 1.3 μm band.

[0162] Accordingly, the present invention contributes to providing a lowcost optical fiber with excellent dependability that is able to be laidwith a small bending diameter or that allows surplus optical fiber to bestored in a small diameter, to say nothing of the arterial systems, inan access system, namely, in an office or inside the home.

What is claimed is:
 1. An optical fiber comprising: a core that has asubstantially uniform first refractive index and a cladding that islocated outside the core and that has a substantially uniform secondrefractive index, wherein: an outer diameter of the core is in a rangeof 4 to 8 μm, and a relative refractive index difference between thefirst refractive index and second refractive index when secondrefractive index is taken as a reference is in a range of 0.4 to 0.8%; achromatic dispersion at a wavelength of 1550 nm is in a range of 2 to 15ps/nm/km; and an effective area at a wavelength of 1550 nm is 40 μm² ormore.
 2. The optical fiber according to claim 1, wherein the outerdiameter of the core is in a range of 4.5 to 5 μm, and the chromaticdispersion is in a range of 2 to 6 ps/nm/km.
 3. The optical fiberaccording to claim 1, wherein the outer diameter of the core is in arange of 5 to 8 μm, and the chromatic dispersion is in a range of 6 to15 ps/nm/km.
 4. The optical fiber according to claim 1, wherein theouter diameter of the core is in a range of 5 to 6.5 μm, and thechromatic dispersion is in a range of 6 to 10 ps/nm/km.
 5. The opticalfiber according to claim 1, wherein the relative refractive indexdifference is in a range of 0.4 to 0.6%, and the effective area is 50μm² or more.
 6. The optical fiber according to claim 5, wherein therelative refractive index difference is in a range of 0.4 to 0.5%, andthe effective area is 60 μm² or more.
 7. The optical fiber according toclaim 1, wherein transmission loss is 0.35 dB/km or less over awavelength range from 1360 to 1400 nm.
 8. The optical fiber according toclaim 1, wherein transmission loss is 0.40 dB/km or less over awavelength range from 1260 to 1625 nm.
 9. The optical fiber according toclaim 4, wherein V_(core) is 15% μm² or less, where, V_(core) isobtained by multiplying π by a value obtained by integrating a sum of rand Δn (r) with r being in a range of 0 to r_(core), Δn (r) being therelative refractive index difference, and r_(core) being the outermostradius of the core at a radius r of the optical fiber.
 10. The opticalfiber according to claim 9, wherein the relative refractive indexdifference is in a range of 0.4 to 0.6%, and the effective area is 50μm² or more.
 11. The optical fiber according to claim 3, wherein therelative refractive index difference is in a range of 0.51 to 0.59%, andthe core diameter is in a range of 5.5 to 7.0 μm.
 12. The optical fiberaccording to claim 11, wherein when the comparative refractive indexdifference is Δ, V_(core) is greater than (−17.25·Δ+25.2) and less than20% μm², where V_(core) is obtained by multiplying π by a value obtainedby integrating a sum of r and Δn (r) with r being in a range of 0 tor_(core), Δn (r) being the relative refractive index difference, andr_(core) being the outermost radius of the core at a radius r of theoptical fiber.
 13. The optical fiber according to claim 12, wherein theoptical fiber has a mode field diameter (MFD) of 7.8 μm or greater at awavelength of 1550 nm, and a bending loss of 0.3 dB/m or less at atolerable bending diameter of 20 mm.
 14. The optical fiber according toclaim 13, wherein connection loss with a normal single mode opticalfiber at ITU-T G.652 Recommendation is 0.35 dB or less at a wavelengthof 1550 nm.
 15. The optical fiber according to claim 9, whereintransmission loss is 0.35 dB/km or less over a wavelength range from1360 to 1400 nm.
 16. The optical fiber according to claim 9, whereintransmission loss is 0.40 dB/km or less over a wavelength range from1260 to 1625 nm.
 17. An optical transmission path comprising the opticalfiber as claimed in claim
 1. 18. The optical transmission path accordingto claim 17 further comprising a dispersion compensator that is combinedwith the optical fiber.
 19. The optical fiber according to claim 1,wherein the optical fiber comprises silica glass as a base materialthereof.
 20. The optical fiber according to claim 1, wherein the opticalfiber has a two layer structure formed by the core and the cladding.